This invention relates to a method of inhibiting coke formation in pyrolysis furnaces. Specifically, this invention relates to a method of inhibiting coke formation in ethylene dichloride/vinyl chloride pyrolysis furnaces.
Thermal pyrolysis or cracking of ethylene dichloride (EDC) to vinyl chloride (VC) is the major industrial process for vinyl chloride monomer (VCM) production at present. The thermal pyrolysis process entails the use of pyrolysis furnaces, also known as EDC-VCM furnaces, to thermally convert EDC to VC. The pyrolysis process occurs as a homogeneous, first-order, free-radical chain reaction. The general reaction mechanism involves the following steps: 
A typical pyrolysis furnace has three sequential building blocks: convection section, radiant section, and transfer line exchanger (TLE). Metal alloy serpentine coils run through the convection section and the radiant section, and connect to the TLE. The convection section utilizes the convection heat from the radiant section to preheat, and sometime to vaporize and preheat EDC feed. The coils in the radiant section function as the pyrolysis reactor where the preheated EDC feed is cracked to VC.
Because of the severe operation environment in the pyrolysis furnaces, iron alloys of high Ni and Cr content are common materials of construction of the pyrolysis furnaces.
The TLE is a heat exchange device, which quickly quenches the effluent from the pyrolysis reactor. The quenching is to stop any product degradation under adiabatic condition in the past furnace zone.
Industrial pyrolysis reactors are typically operated at temperatures of from about 470xc2x0 C. to about 550xc2x0 C. (about 878xc2x0 F. to 1022xc2x0 F.), at gauge pressures of from about 1.4 Mpa to about 3.0 Mpa (about 200 psig to about 435 psig) and with a residence time from about 2 seconds to about 30 seconds. EDC conversion per pass through a pyrolysis furnace is normally maintained around 50-55% with a selectivity of 96-99% to vinyl product. VC and HCl are the major components in the pyrolysis furnace effluent. By-products from the pyrolysis process range from the very lights, such as methane, acetylene, ethylene and methyl chloride, to the heavies, such as carbon tetrachloride, trichloroethane and solid carbonaceous material. Solid carbonaceous material is usually referred to as coke, and coke is an unwanted byproduct of the pyrolysis process.
Higher conversion in the pyrolysis process is, in most cases, desired. However, increasing cracking severity beyond conventional operation conditions generally leads to only a small increase in EDC conversion at the expense of the selectivity to vinyl chloride product. Furthermore, any outstanding increase in cracking severity causes a drastic increase in coke formation and a sharp drop in VC selectivity.
Fouling of the pyrolysis furnace and the TLE occurs due to formation of coke. In fact, coke formation often becomes the major limitation in pyrolysis furnace operation and VC production. Formation of coke with resultant fouling decreases the effective cross-sectional area of the process feed flow through the pyrolysis reactor and the TLE, and thus increases the pressure drop across pyrolysis furnaces. In order to compensate for the pressure buildup, generally, a reduction in EDC feed rate is necessary. A reduction in EDC feed rate means an overall reduction in production. Another undesirable feature of coke formation is that the coke is a good thermal insulator, and thus coke formation reduces the heat transfer across the walls of the pyrolysis reactor. The reduction in heat transfer requires a gradual increase in furnace firing duty to maintain the cracking reactions at a desired conversion level. Furnace fire duty thus can also become the limiting factor for conversion and overall VC production. To maintain the capacity and the fire efficiency of pyrolysis furnaces at optimum levels, pyrolysis operation has to periodically cease for coke removal (decoke), which causes production down time.
Known methods for the removal of coke from pyrolysis furnaces include controlled combustion or mechanical cleaning, or a combination of both methods. In the combustion process, a mixture of steam and air of various steam/air ratios is admitted in the pyrolysis furnace at an elevated temperature, and the coke in the reactor is burnt out under a controlled condition. This process is conventionally referred as hot decoke. For the mechanical cleaning, coke is physically chipped off the pyrolysis reactor inner surface and removed from the reactor. Both cracking and the hot decoke operations expose the pyrolysis furnace to a cycle between a HCl and chlorinated hydrocarbon-rich reducing environment and an oxygen-rich oxidizing environment at elevated temperatures, which causes corrosion and degradation of the pyrolysis furnace and shortens the reactor lifetime. Therefore, methods of prevention of coke formation are desired in order to improve vinyl chloride production and avoid the coke removal operation.
Great Britain Patent No. 1,494,797, VINYL CHLORIDE BY A DEHYDROCHLORINATION PROCESS, teaches a method of addition of 200-5000 PPM of 1,1,2-trichloroethane to reduce coke formation in EDC-VCM pyrolysis furnaces.
U.S. Pat. No. 3,896,182 teaches a method of reducing coke formation and fouling by lowering the oxygen content in the EDC feed.
Coke formation in pyrolysis furnaces continues to be undesirable and thus effective alternative methods to reduce the formation of coke in pyrolysis furnaces are always desired.
A method of reducing the formation of coke deposits on the heat-transfer surfaces of an ethylene dichloride to vinyl chloride pyrolysis furnace comprising exposing the heat transfer surfaces of said pyrolysis furnace to a phosphite selected from the group consisting of phosphites with the general formula: 
wherein A1, A2 and A3 are selected from the group consisting of xe2x80x94OR1, xe2x80x94SR2 and Cl, wherein R1 and R2 are selected from the group consisting of alkyl, aryl, alkylaryl and arylalkyl, wherein A1, A2 and A3 may be the same or different, provided that at least one of A1, A2, and A3 is not Cl.